In audio reproduction much effort has been expended in improving the faithful reproduction of an original event or performance. Every link in the reproduction chain carries the full responsibility for the integrity of the sound. Each component in the reproduction chain is scrutinized for flaws in handling the sonic information. The listening environment is a major component in this chain and is one of the most easily degraded components.
When considering just the amplitude and phase response distortion, a modern recording and playback system can deliver a signal from a microphone through the recording playback process to the output of an amplifier with less than two decibels (dB) of amplitude variation over the audio band. Loudspeakers display considerably less accuracy, but several still manage to deliver less than five dB variation at all but the lowest frequencies. A reasonably live rectangular room, however, is easily capable of amplitude variations of 20 dB, corresponding to an energy error of 10,000%.
These amplitude variations found in standard rooms skew the original amplitude and phase relations of the music and cause overhang and "boominess" in somewhat the same way as a poorly designed speaker enclosure distorts sound when driven by an amplifier with a very low damping factor. At frequencies below 200 Hz, the average listening room begins to behave like a classic rectangular chamber and exhibits large response variation due to standing waves. Standing waves develop between opposing corners and parallel surfaces where pressure can build up. The gravest of these resonances corresponds to the longest dimension of the room. In this resonance mode, an acoustic standing wave develops high cumulative energy with high pressure in the corners and high air velocity in the center of the room. Air cannot flow through the walls and pressure builds up in the corners much as it does in the throat of a horn. Not having a rectangular room does not mean that the resonant peaks do not develop.
Prior art solutions to this problem of standing wave resonance in listening rooms center around passive designs. One method would be to design a room wherein the worst resonant modes are avoided. However this would be impractical for existing rooms, and may be costly for new structures. A number of efforts have involved completely lining the room with absorbent foam, fiberglass or cloth to eliminate reflected sound, but this is not effective at the lower frequencies where the worst of the room variations exists. Even if it were possible to create a completely nonreflective environment, it would not solve the problem. Listening to audio in an anechoic chamber or dead room is not a very satisfying experience. Nor is it practical to line all of the surfaces of a room with sound absorbent materials.
In U.S. Pat. No. 2,160,638, Bedell et al. discloses a sound absorbing unit employing sound absorbing materials. The sound absorbing unit comprises a large thin perforated metal casing containing highly efficient sound absorbing material and is adapted to be mounted with both sides of the material exposed to the sound waves in a room. The perforations in the metal are small enough to be inconspicuous and are of such spacing as to make the casing substantially acoustically transparent.
In U.S. Pat. No. 2,502,020 Olson discloses an acoustic absorber having a casing which encloses a large volume of air. The wall structure of the casing is made up of a material which is previous to sound waves and which offers a high impedance thereto. The preferred material used for the casing is fiberglass. The acoustic absorber of Olson may be cylindrical in shape.
Active sound wave pressure reducers are known. In a book entitled "Acoustical Engineering" by H. F. Olson, D. Van Nostrand Co., Inc., pp. 415-417 and 511, there is a teaching of an active sound absorber featuring a microphone, amplifier and loudspeaker connected to reduce the sound pressure of acoustic waves in the vicinity of the microphone-loudspeaker.
It is an object of the present invention to devise a low frequency acoustic absorber which reduces the low frequency resonance modes typical of a rectangular room.
It is another object of the invention to improve the quality of a reproduced audio event on a playback system in a generally rectangular room.